In semiconductor devices the precise location of the P-N junction, for example its location relative to a particular interface within the device, is often critical to the device performance. The location of the junction may vary from one batch of devices to another because of fluctuations in, or inadequate control of, the conductivity modifier concentration, the diffusion time or temperature, and subsequent processing steps. These variations can result in a low yield of useful devices at the end of the manufacturing process.
A particular example of this problem arises in the manufacture of a P-I-N photodetector such as that described by Webb et al. in the IEEE Transactions on Electron Devices ED-30, 395(1983). This photodetector includes an N-type InP substrate, an In.sub.0.53 Ga.sub.0.47 As absorptive layer on the substrate and a P-type InP cap layer on the absorptive layer. The as-deposited absorptive layer is undoped and generally contains a low number of N-type conductivity modifiers. The P-N junction is formed in the absorptive layer, preferably a small distance from the interface between the absorptive and cap layers, by diffusion of P-type conductivity modifiers from the cap layer into the absorptive layer during the deposition of the cap layer. Variations in the growth procedure, for example too large a concentration of P-type conductivity modifiers in the cap layer will convert a large portion of the absorptive layer to P-type conductivity. It is also useful to provide a comparatively thick cap layer over the absorptive layer to increase the resistance of the detector to mechanical damage during wire bonding. However, during the prolonged high temperature deposition of the cap layer the conductivity modifiers can diffuse through most or all of the absorptive layer.
The effect of this larger than desired P-type region is that a large fraction of the light passing through the transparent cap layer is then absorbed in the P-type region of the absorptive layer. The electron-hole pairs so-generated recombine in the P-type portion before the electrons and holes can be separated and detected, thereby rendering the device useless.
It would be desirable to have a detector structure and method of fabrication which reduces or eliminates this problem and which provides improved control during manufacture of the location of the P-N junction.